In fabricating lightguide fiber for communication purposes, it is well known to position a cylindrical glass preform in an open-ended furnace in order to form a melt zone at an end of the preform. A lightguide fiber is then pulled from the melt zone, by a drawing device, and is further gathered onto a rotating take-up spool. There is a fiber catenary between the drawing motor and the take-up spool to permit the collection of the fiber at a low tension in order to minimize damage to the fiber or any coating thereon and to make accurate optical measurements.
Such a technique requires that a relationship exist between the rim velocities of the drawing and the take-up devices so that the initial sag placed in the fiber catenary between the two devices does not substantially change in depth during the fiber drawing-collecting process. This may be readily accomplished by a catenary controller described in detail in U.S. Pat. No. 4,195,791 which issued on Apr. 1, 1980 to R. E. Frazee, assigned to the instant assignee and is hereby incorporated by reference herein. The catenary controller of the Frazee patent uses a closed circuit television (CCTV) camera to monitor the sag of the fiber between the drawing device and the take-up spool. The output from the CCTV camera is forwarded to a video signal processor which continuously determines the displacement of the lowest point of the fiber catenary loop from an optimum position and generates an electrical signal proportional thereto. The electrical signal is forwarded to a spooling motor to adjust the speed of the take-up spool to cause the fiber catenary to return to a predetermined position.
Such a technique has been found to be most effective for maintaining the position of the catenary loop within the predetermined limits which desirably results in low take-up tension. However, problems arise when it is necessary to change from a full take-up reel to an empty take-up reel as the fiber continues to be drawn because the catenary loop tends to fall outside the field of view of the CCTV camera at that time. This results in the loss of control of the spooling process with the attendant loss of fiber.
Presently such a transfer is accomplished by placing an empty spool proximate the full spool and in axial alignment therewith in such a manner that both spools rotate at the same velocity. When the transfer from the full to empty spool occurs, a substantially constant tangential velocity of the fiber must be maintained, and therefore a nearly instantaneous change in angular velocity of the take-up spool must occur. It has been calculated that at a draw speed of one meter per second the catenary control cicuitry has one second to change the angular velocity of the take-up motor to maintain control. However, at five meters per second the dynamic response of the system must be such that the adjustment is made in less than 0.2 second.
It has been empirically determined that with an operator assisting, transfers from full to empty spools can be reliably accomplished at fiber draw speeds up to at least two meters per second while maintaining the catenary loop within the field of view of the CCTV camera. Successful transfers become more difficult to achieve as the draw speed increases and are virtually impossible at draw speeds much above three meter per second.
Accordingly, there is a need for dynamically transferring a lightguide fiber from a full to an empty reel at high speed with reliable control while spooling the fiber at a constant velocity.